Singlet oxygen oxidation of organic substrates

ABSTRACT

The invention relates to a method for oxidizing organic substrates using  1 O 2  in which hydrophobic organic substrates that react with  1 O 2 are added to an organic solvent in the presence of a heterogeneous or homogeneous catalyst with 30–70% being compr Afterwards, H 2 O 2  is catalytically decomposed into water and  1 O 2 , and the oxidation into corresponding oxidation products ensues.

The only singlet oxygen oxidation (¹O₂—Ox) which is currently carried out industrially is the photochemical ¹O₂—Ox in which the ¹O₂ is generated by a photochemical route. The disadvantage of this process is given by the high costs of the photochemical equipment required, and by a limited service life. The required lamps degenerate relatively rapidly during the oxidation as a result of soiling of the glass surface. In addition, this process is not suitable for colored substrates. The process is actually suitable only for fine chemicals which are prepared on a relatively small scale. (La Chimica e l'Industria, 1982, Vol. 64, page 156).

For this reason, attempts have been made to find other process variants for the ¹O₂—Ox which are suitable for the ¹O₂—Ox of non-water-soluble, hydrophobic organic substrates.

J. Am. Chem. Soc., 1968, 90, 975 describes, for example, the classical “dark” ¹O₂—Ox in which ¹O₂ is not generated photochemically, but chemically. In this process, hydrophobic substrates are oxidized by means of a hypochlorite/H₂O₂ system in a solvent mixture of water and organic solvent. However, this process has only found a few synthetic applications since many substrates are only sparingly soluble in the required medium. In addition, the use possibility is rather limited because of secondary reactions between hypochlorite and substrate or solvent. In addition, a large part of the ¹O₂ is deactivated in the gas phase. In addition, this process is not suitable for industrial scale since in the organic medium addition of the hypochlorite onto H₂O₂ results, and a large excess of H₂O₂ is required to suppress the secondary reaction of substrate with hypochlorite. An additional disadvantage arises as a result of the formation of stoichiometric amounts of salt.

A variant of the “dark” ¹O₂—O_(x), which is not based on hypochlorite and thus should partly avoid the above disadvantages, is known, for example, from J. Org. Chem., 1989, 54, 726 or J. Mol. Cat., 1997, 117, 439, according to which some water-soluble organic substrates are oxidized with H₂O₂ and a molybdate catalyst in water as solvent. According to Membrane Lipid Oxid. Vol. II, 1991, 65, the ¹O₂—Ox of water-insoluble, organic substrates with the molybdate/H₂O₂ system is difficult since it was assumed that none of the customary solvents is suitable for maintaining the disproportionation, catalyzed by molybdate, of H₂O₂ into water and ¹O₂. As is described in Membrane Lipid Oxid. Vol. II, 1991, 65, water-insoluble substrates, such as, for example, α-terpinene or β-citronellol, can be oxidized with the molybdate/H₂O₂ system in a MeOH/water mixture (70/30) with only relatively low yields of 70%. As well as the only moderate yields which are obtained using MeOH/water, the very small application spectrum of this method based on aqueous solvent mixtures is an additional disadvantage since this method, as is clear from J. Am. Chem. Soc., 1997, 119, 5286, is limited to somewhat hydrophilic substrates or to hydrophobic substrates with a low molecular weight.

J. Am. Chem. Soc., 1997, 119, 5286 and EP-A-0 288 337 describes a process that permits the generation, catalyzed by molybdate in aqueous solution, of ¹O₂ from H₂O₂ and is nevertheless suitable for hydrophobic substrates from a relatively wide molecular weight range. In this process, a microemulsion is used as reaction medium. However, use on an industrial scale is associated with problems since product isolation from the microemulsion is difficult. In addition, it is a relatively expensive process since rather large amounts of surfactant have to be used relative to the substrate.

Accordingly, it was an object of the present invention to find an improved method of the “dark” ¹O₂—Ox which can be used simply, cost-effectively and in an environmentally friendly manner on an industrial scale and is suitable for a large number of substrates.

Unexpectedly, it has now been found that the “dark” ¹O₂—Ox can be carried out in an extremely efficient manner with high yield in certain organic solvents as reaction medium, without the addition of water as cosolvent and without surfactant.

Accordingly, the present invention provides a process for the oxidation of organic substrates by means of ¹O₂, which comprises adding 30–70% strength H₂O₂ to hydrophobic organic substrates which react with ¹O₂ in an organic solvent in the presence of a heterogeneous or homogeneous catalyst, whereupon, following the catalytic decomposition of H₂O₂ to give water and ¹O₂, oxidation to give the corresponding oxidation products takes place.

The process according to the invention is suitable for the oxidation of hydrophobic organic substrates which react with ¹O₂.

Accordingly, substrates which may be used are the following compounds: olefins which contain one or more, i.e. up to 10, preferably up to 6, particularly preferably up to 4, C═C double bonds; electron-rich aromatics, such as C₆–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, phenols, polyalkylbenzenes, polyalkoxybenzenes; polycyclic aromatics having 2 o 10, preferably up to 6, particularly preferably up to 4 aromatic rings; sulfides, such as, for example, alkyl sulfides, alkenyl sulfides, aryl sulfides which are either mono- or disubstituted on the sulfur atom, and heterocycles having an O, N or S atom in the ring, such as, for example, C₄–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, furans, C₄–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, pyrroles, C₄–C₆₀, preferably up to C₃₀, particularly preferably up to C₂₀, thiophenes. In this connection, the substrates may have one or more substituents, such as halogen (F, Cl, Br, I), cyanide, carbonyl groups, hydroxyl groups, C₁–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, alkoxy groups, C₁–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, alkyl groups, C₆–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, aryl groups, C₂–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, alkenyl groups, C₂–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, alkynyl groups, carboxylic acid groups, ester groups, amide groups, amino groups, nitro groups, silyl groups, silyloxy groups, sulfone groups, sulfoxide groups. In addition, the substrates may be substituted by one or more NR¹R² radicals in which R¹ or R² may be identical or different and are H; C₁–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, alkyl; formyl; C₂–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, acyl; C₁–C₅₀, preferably up to C₃₀, particularly preferably up to C₂₀, benzoyl, where R¹ and R² may also together form a ring, such as, for example, in a phthalimido group.

Examples of suitable substrates are: 2-butene; isobutene; 2-methyl-1-butene; 2-hexene; 1,3-butadiene; 2,3-dimethylbutene; A^(9,10)-octalin, 2-phthalimido-4-methyl-3-petene; 2,3-dimethyl-1,3-butadiene; 2,4-hexadiene; 2-chloro-4-methyl-3-pentene; 2-bromo-4-methyl-3-pentene; 1-trimethylsilylcyclohexene; 2,3-dimethyl-2-butenyl-para-tolylsulfone; 2,3-dimethyl-2-butenyl-para-tolyl sulfoxide; N-cyclohexenylmorpholine; 2-methyl-2-norbornene; terpinolene; α-pinene; β-pinene; β-citronellol; ocimene, citronellol; geraniol; farnesol; terpinene; limonene; trans-2,3-dimethylacrylic acid; α-terpinene; isoprene; cyclopentadiene; 1,4-diphenylbutadiene; 2-ethoxybutadiene; 1,1′-dicyclohexenyl; cholesterol; ergosterol acetate; 5-chloro-1,3-cyclohexadiene; 3-methyl-2-buten-1-ol; 3,5,5-trimethylcyclohex-2-en-1-ol; phenol, 1,2,4-trimethoxybenzene, 2,3,6-trimethylphenol, 2,4,6-trimethylphenol, 1,4-dimethylnaphthalene, furan, furfuryl alcohol, furfural, 2,5-dimethylfuran, isobenzofuran, dibenzyl sulfide, (2-methyl-5-tert-butyl)phenyl sulfide, etc.

As a result of the oxidation according to the invention, the corresponding oxidation product is obtained from the substrates. Alkenes, (polycyclic) aromatics or heteroaromatics give, in particular, hydroperoxides or peroxides which are able to further react under the reaction conditions to give alcohols, epoxides, acetals or carbonyl compounds, such as ketones, aldehydes, carboxylic acids or esters, if the hydroperoxide or the peroxide is unstable.

The oxidation according to the invention is carried out in an organic solvent.

Suitable solvents are C₁–C₈-alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol; ethylene glycol, propylene glycol, formamide, N-methylformamide, dimethylformamide, sulfolane, propylene carbonate.

Preference is given to using methanol, ethanol, propanol, isopropanol, ethylene glycol, propylene glycol, formamide, N-methylformamide or dimethylformamide, particularly preferably methanol, ethanol, ethylene glycol, propylene glycol, formamide or dimethylformamide as solvent.

Up to 25% of water may optionally be added to the organic solvent. However, the addition of water does not bring any advantages for the reaction. Water is therefore preferably not added.

A metal which is suitable for ¹O₂ oxidations and is described, for example, in J. Am. Chem. Soc., 1985, 107, 5844 or in Membrane Lipid Oxid. Vol. II, 1991, 65, is added as heterogeneous or homogeneous inorganic catalyst to the solvent/substrate mixture.

In this connection, the metal can be in forms customary for ¹O₂ oxidations, for example as the oxide, oxo complex, nitrate, carboxylate, hydroxide, carbonate, chloride, fluoride, sulfate, tetrafluoroborate, etc. Preference is given to catalysts based on molybdenum, tungsten, scandium, vanadium, titanium, zirconium, praseodymium, neodymium, samarium, europium, terbium, dysprosium, holmium, erbium, ytterbium and lutetium. Particular preference is given to molybdenum catalysts.

The amount of catalyst used depends on the substrate used and is between 1 and 50 mol %, preferably between 5 and 25 mol %.

This is followed by the addition of 30–70% strength, preferably 40–60% strength, H₂O₂. H₂O₂ is preferably added slowly or in portions to the reaction mixture of solvent, substrate and catalyst, the reaction mixture being stirred.

In the process according to the invention, the consumption of H₂O₂ is dependent on the substrate used. For reactive substrates, 2 to 3 equivalents of H₂O₂ are preferably needed, while less reactive substrates are preferably reacted with 3 to 10 equivalents of H₂O₂.

The reaction temperature is between 0 and 50° C., preferably between 15 and 35° C.

In some cases, to improve activation of the catalyst, it may be advantageous to add customary basic or acidic additives to the reaction mixture.

The course of the reaction can be monitored by means of UV spectroscopy or by means of HPLC. When the reaction is complete, i.e. after 1 to 30 hours, depending on the reaction conditions, the reaction mixture is worked up. Removal of the catalyst has proven unexpectedly simple particularly when molybdate catalysts are used, such as e.g. sodium molybdate, in some solvents. Although the reaction proceeds completely homogeneously when molybdate catalysts, such as, for example, Na₂MoO₄.2H₂O in mono-hydroxylic, alcoholic solvents, i.e. in monohydric alcohols such as, for example, methanol or ethanol, are used, provided H₂O₂ is added, the catalyst, after all of the H₂O₂ has been added, precipitates out of the reaction mixture, as a result of which the catalyst can be separated off by simple centrifugation or filtration, and recycled.

The end-product which remains can, where appropriate, be purified by means of recrystallization, extraction or distillation.

The process according to the invention permits the oxidation of a large number of hydrophobic compounds and is particularly advantageous for the oxidation of water-insoluble substrates which could not be oxidized with hitherto known chemical methods with high yield. Accordingly, the process according to the invention is particularly suitable for the oxidation of unsaturated organic compounds, such as terpenes, for example α-terpinene and citronellol, aromatic polycycles, steroids, furans, cyclopentadienes, phenols etc., and generally for all compounds which react with ¹O₂.

The process according to the invention gives the desired end-products in high yields of up to 100% with high purity.

The process according to the invention is characterized by the simple process regime which is best suited to the industrial scale since it can take place in simple multipurpose plants and with simple work-up steps, and can be used for a wide spectrum of substrates.

EXAMPLE 1

-   a) 0.2 ml of an aqueous 2 mol/l Na₂MoO₄ solution or -   b) 0.4 mmol of Na₂MoO₄.2eq.     were added to a thermostated solution (25° C.) of 2 mmol of an     organic substrate (α-terpinene or β-citronellol) in 4 ml of an     organic solvent. 0.08 ml of H₂O₂ (50%) were added to this mixture.     After the reaction mixture had turned yellow again, two further 0.08     ml portions of H₂O₂ (50%) were added. The course of the reaction was     monitored in the case of α-terpinene by means of UV spectroscopy     (266 nm) and in the case of β-citronellol by means of HPLC (MeOH/H₂O     70/30).

EXAMPLE 2

-   a) 0.2 ml of an aqueous 2 mol/l Na₂MoO₄ solution or -   b) 0.4 mmol of Na₂MoO₄.2eq.     were added to a thermostated solution (25° C.) of 2 mmol of an     organic substrate (α-terpinene or β-citronellol) in 4 ml of an     organic solvent. 0.25 ml of H₂O₂ (50%) were added to this mixture in     one portion. The course of the reaction was monitored in the case of     α-terpinene by means of UV spectroscopy (266 nm) and in the case of     β-citronellol by means of HPLC (MeOH/H₂O 70/30).

The solvents used and the conversion of α-terpinene into ascaridol and β-citronellol into a 1/1 mixture of the corresponding hydroperoxides are given in table 1:

TABLE 1 Cat. a) Example Substrate Solvent or b) Conversion 1 α-terpinene Methanol a 100% after 2 h 1 α-terpinene Methanol b >95% after 2 h 2 α-terpinene Methanol a >95% after 2 h 1* α-terpinene Ethanol a 90% after 22 h 1 α-terpinene Formamide a 70% after 21 h 1 α-terpinene N-Me-formamide a 75% after 3 h 1 α-terpinene DMF a 64% after 21 h 1 α-terpinene Sulfolane a 70% after 21 h 1 β-citronellol Methanol a 80% after 3 h 1 β-citronellol Formamide a 95% after 3 h *3rd portion of H₂O₂ was 0.09 ml

EXAMPLE 3 Product Isolation from Reaction Mixture with Methanol as Solvent

After the time given in table 1, the precipitated-out catalyst was removed from the reaction mixture by means of centrifugation. The precipitate was washed twice with absolute ethanol, and the combined solvent batches (methanol and ethanol) were removed on a rotary evaporator. The oxidation product which remained was dissolved in CDCl₃ for NMR analysis. In the case of α-terpinene, analysis confirmed the formation of a virtually quantitative amount of >95% pure ascaridol. In the case of β-citronellol, approximately 80% of product were obtained which, according to NMR analysis, consisted of a 1/1 mixture of the two corresponding hydroperoxides.

EXAMPLE 4

At 35° C., 45 μl of H₂O₂ (50%) were added to a solution of 325 μl of α-terpinene and 48.5 mg of Na₂MoO₄.2H₂O in 4 ml of methanol. Five further 45 μl portions of H₂O₂ (50%) were added to this mixture as soon as the red-colored reaction mixture turned yellow again. After 1.5 hours, the reaction mixture was analyzed by means of HPLC. Analysis gave a quantitative formation of ascaridol.

EXAMPLE 5

At 25° C., 80 μl of H₂O₂ (50%) were added to a solution of 365 μl of citronellol and 97 mg of Na₂MoO₄.2H₂O in 4 ml of ethylene glycol. After 1, 2 and after 19 hours, 3 further 80 μl portions of H₂O₂ (50%) were added to this mixture. HPLC analysis gave a 100% conversion with a yield of secondary hydroperoxide of 38% and a yield of tertiary hydroperoxide of 62%. 

1. A process for the oxidation of organic substrates by means of ¹O₂, which consists of adding 40–60% strength H₂O₂ to hydrophobic organic substrates which react with ¹O₂ in a monohydric C₁–C₈ alcohol as a solvent in the presence of 5–25 mol % of a homogeneous molybdate catalyst, whereupon H₂O₂ is catalytically decomposed to give water and ¹O₂, oxidizing said substrate to the corresponding oxidation products with precipitation of the catalyst, removing said precipitated catalyst by centrifugation or filtration and recycling said catalyst to said oxidation.
 2. The process as claimed in claim 1, wherein the substrates which react with ¹O₂ are olefins which contain 1 to 10 C═C double bonds; C₆–C₅₀ phenols, polyalkylbenzenes, polyalkoxybenzenes; polycyclic aromatics having 2 to 10 aromatic rings; alkyl sulfides, alkenyl sulfides, aryl sulfides which are either mono- or disubstituted on the sulfur atom, and C₄–C₆₀ heterocycles having an O, N or S atom in the ring, which may be unsubstituted or may be mono- or polysubstituted by halogens, cyanide, carbonyl groups, hydroxyl groups, C₁–C₅₀ alkoxy groups, C₁–C₅₀ alkyl groups, C₆–C₅₀ aryl groups, C₂–C₅₀ alkenyl groups, C₂–C₅₀ alkynyl groups, carboxylic acid groups, ester groups, amide groups, amino groups, nitro groups, silyl groups, silyloxy groups, sulfone groups, sulfoxide groups or by one or more NR¹R² radicals in which R¹ or R² may be identical or different and are H; C₁–C₅₀ alkyl; formyl; C₂–C₅₀ acyl, C₇–C₅₀ benzoyl, where R¹ and R² may also together form a ring.
 3. The process of claim 1, wherein the reaction temperature is between 0 and 50° C.
 4. The process of claim 2 wherein the reaction temperature is 15 to 35° C.
 5. The process of claim 1 wherein 2 to 10 equivalents of H₂O₂ are used depending on the substrate used. 